The work foreshadowed in this letter appeared in the summer of 1710, as the “First part” of a _Treatise concerning the Principles of Human Knowledge, wherein the chief causes of error and difficulty in the Sciences, with the grounds of Scepticism, Atheism, and Irreligion, are inquired into_. In this fragment of a larger work, never finished, Berkeley’s spiritual conception of matter and cosmos is unfolded, defended, and applied. According to the _Essay on Vision_, the world, as far as it is visible, is dependent on living mind. According to this book of _Principles_ the whole material world, as far as it can have any practical concern with the knowings and doings of men, is real only by being realised in like manner in the percipient experience of some living mind. The concrete world, with which alone we have to do, could not exist in its concrete reality if there were no living percipient being in existence to actualise it. To suppose that it could would be to submit to the illusion of a metaphysical abstraction. Matter unrealised in its necessary subordination to some one’s percipient experience is the chief among the illusions which philosophers have been too ready to encourage, and which the mass of mankind, who accept words without reflecting on their legitimate meanings, are ready to accept blindly. But we have only to reflect in order to see the absurdity of a material world such as we have experience of existing without ever being realised or made concrete in any sentient life. Try to conceive an eternally dead universe, empty for ever of God and all finite spirits, and you find you cannot. Reality can be real only in a living form. Percipient life underlies or constitutes all that is real. The _esse_ of the concrete material world is _percipi_. This was the “New Principle” with which the young Dublin Fellow was burdened—the Secret of the universe which he had been longing to discharge upon mankind for their benefit, yet without sign of desire to gain fame for himself as the discoverer. It is thus that he unfolds it:— “Some truths there are so near and obvious to the mind that a man need only open his eyes to see them. Such I take this important one to be, viz. that all the choir of heaven and furniture of the earth, in a word, all those bodies which compose the mighty frame of the world, have not any subsistence without a Mind; that their _being_ is to be perceived or known; that consequently so long as they are not actually perceived by me, or do not exist in my mind, or that of any other created spirit, they must either have no existence at all, or else subsist in the mind of some Eternal Spirit: it being perfectly unintelligible, and involving all the absurdity of abstraction, to attribute to any single part of them an existence independent of a Spirit(3).”